1. Field of the Invention
The present invention relates to a sealing film for a Solar cell which mainly consists of ethylene-vinyl acetate copolymer and a solar cell prepared by using the film, in particular, to the sealing film for a Solar cell and the solar cell which have excellent yellowing resistance.
2. Description of the Related Art
A solar cell has been paid attention as a device directly converting solar energy into electric energy from the viewpoints of effective use of natural resources and prevention of environment deterioration, and various solar cells have been developed.
In a solar cell, as described in FIG. 1, photovoltaic elements 4 made of silicon etc. are sealed by the sealing films 3A, 3B made of EVA (ethylene-vinyl acetate copolymer) film between a glass plate 1 as a transparent front surface (light receiving surface) protection member and a rear surface protection member 2 (backside covering material). A sealing film placed at a light-receiving surface side of the cells is referred to “surface sealing film”, and a sealing film placed at a rear surface side of the cells is referred to “rear surface sealing film”.
A solar cell is prepared by the following steps: a glass plate 1, surface EVA film for sealing 3A, silicon photovoltaic elements 4, rear surface EVA film for sealing 3B and a backside covering material 2 are superposed in this order and cross-linked the EVA by heating and pressing to unify the superposed product, whereby a solar cell can be obtained.
From a viewpoint of the improvement of the power generation efficiency, it is desired strongly to introduce light falling on the solar cell into the photovoltaic elements of the solar cell as effectively as possible. Therefore, it is needed for the EVA film for sealing to have excellent transparent and transmit most of the light without absorbing and reflecting the light. In addition, when the solar cell is used for a long time, there are problems that the transmission of the light may be reduced and the defective appearance is caused by turning the EVA film for sealing yellow because of the influence of light and heat.
In order to prevent such yellowing, for example various ultraviolet absorbers and light stabilizers are used in the EVA film for sealing. For example, a patent document 1 (JP-A-H07 (1995)-169986) discloses that 2,5-dimethyl-2,5-di(t-butylperoxy)hexane falling into peroxyester peroxide is used as organic peroxide (cross-linking agent), and 2-hydroxy-4-n-octoxybenzophenone falling into monohydroxyalkoxybenzophenone type is used as the ultraviolet absorber. In addition, a patent document 2 (JP-A-2000-183381) discloses that 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane is used as organic peroxide and 2-hydroxy-4-n-octoxybenzophenone, which is the same as above, is used as ultraviolet absorber.